Fabricating patterns on the surface of devices may confer new or improved properties and functions to said devices. Thin-films of alkali halides may exhibit self-assembly properties that may be employed to create patterns at the surface of layers.
Photovoltaic devices are generally understood as photovoltaic cells or photovoltaic modules. Photovoltaic modules ordinarily comprise arrays of interconnected photovoltaic cells. A thin-film photovoltaic or optoelectronic device is ordinarily manufactured by depositing material layers onto a substrate. A thin-film photovoltaic device ordinarily comprises a substrate coated by a layer stack comprising a conductive layer stack, at least one absorber layer, optionally at least one buffer layer, and at least one transparent conductive layer stack.
The present invention is also concerned with photovoltaic devices comprising an absorber layer generally based on an ABC chalcogenide material, such as an ABC2 chalcopyrite material, wherein A represents elements in group 11 of the periodic table of chemical elements as defined by the International Union of Pure and Applied Chemistry including Cu or Ag, B represents elements in group 13 of the periodic table including In, Ga, or Al, and C represents elements in group 16 of the periodic table including S, Se, or Te. An example of an ABC2 material is the Cu(In,Ga)Se2 semiconductor also known as CIGS. The invention also concerns variations to the ordinary ternary ABC compositions, such as copper-indium-selenide or copper-gallium-selenide, in the form of quaternary, pentanary, or multinary materials such as compounds of copper-(indium, gallium)-(selenium, sulfur), copper-(indium, aluminium)-selenium, copper-(indium, aluminium)-(selenium, sulfur), copper-(zinc, tin)-selenium, copper-(zinc, tin)-(selenium, sulfur), (silver, copper)-(indium, gallium)-selenium, or (silver, copper)-(indium, gallium)-(selenium, sulfur).
The photovoltaic absorber layer of thin-film ABC or ABC2 photovoltaic devices can be manufactured using a variety of methods such as chemical vapor deposition (CVD), physical vapor deposition (PVD), spraying, sintering, sputtering, printing, ion beam, or electroplating. The most common method is based on vapor deposition or co-evaporation within a vacuum chamber ordinarily using multiple evaporation sources. Historically derived from alkali material diffusion using soda lime glass substrates, the effect of adding alkali metals to enhance the efficiency of thin-film ABC2 photovoltaic devices has been described in much prior art (RUDMANN, D. (2004) Effects of sodium on growth and properties of Cu(In,Ga)Se2 thin films and solar cells, Doctoral dissertation, Swiss Federal Institute of Technology. Retrieved 2014 Apr. 30 from <URL: http://e-collection.ethbib.ethz.ch/eserv/eth:27376/eth-27376-02.pdf>).
Thin-films of alkali halides are known to form island structures (BOUTBOUL, T. et al. On the Surface Morphology of Thin Alkali Halide Photocathode Films. Nuclear Instruments and Methods in Physics Research A 438 (1999) 409-414).
The present invention presents a method to form nanometric patterns, or nanopatterns at the surface of a device, for example an optoelectronic device's thin-film layer, preferably the device's absorber layer. The present invention builds upon a previous filing (PCT/IB2015/056656 claiming priority of 18 Sep. 2014) referred to as “previous filing” in this description. The nanopatterns may comprise rows or arrays of alkali crystals or compounds of alkali crystals formed by deposition of at least one alkali material onto a thin-film layer. The nanopatterns may also comprise rows or arrays of cavities formed by selectively dissolving said alkali crystals and underlying material that underwent a reaction with said alkali crystals. Said nanopatterns are ordinarily formed into a portion of the thickness of the thin-film layer onto which the alkali material is deposited. Said nanopatterns may also be formed into at least one thin-film layer deposited after deposition of said alkali material. The method may advantageously modify at least one thin-film layer's chemical composition, enlarge developed total surface, enlarge developed surface adequate for receiving doping elements, form point contacts with subsequently deposited thin-film layers, form patterns in subsequently deposited thin-film layers, form patterns that confer specific properties to the device, form patterns that confer new functions to the device. The method may also enable tuning of the device through control of pattern properties such as mean area, spatial density, spatial frequency, mean hole size, or area coverage, of cavities, respectively alkali crystals.
In this document, we define alkali metal as being alkali metal in elemental form or precursor thereof.
The present invention exploits adding at least one alkali metal to a layer, for example to the absorber layer of a thin-film optoelectronic device. The invention especially exploits, under control of layer temperature, self-assembly properties of adding the at least one alkali metal to a layer so as to form patterns. For example, adding at least one alkali metal to a layer modifies at least the physical appearance of the surface of the layer and possibly also the chemical content of the layer. Further treating of at least the surface of the absorber layer will modify its physical appearance to reveal concave nanostructures or nanopatterns. Treating of absorber surface may for example be done with a bathing apparatus. The invention discloses independent control of separate alkali metals during adding to layers of the optoelectronic device, the treating of the absorber surface, and the resulting chemical and physical modifications to at least one absorber layer of the optoelectronic device. Effects of the invention on at least one of the device's thin-film layers include at least one of doping, passivation of absorber surface, interfaces, grain boundaries, and defects, elemental interdiffusion, forming of point contacts, forming of nanoholes of controlled dimensions and spatial distribution, modification of layer roughness, optical characteristics, and optoelectronic characteristics such as enhanced open circuit voltage and fill factor. The invention's adding of at least one alkali metal and treating absorber surface enables manufacturing of a thinner optimal buffer layer. In some cases a person skilled in the art may advantageously omit manufacturing the buffer layer. This thinner optimal buffer layer results in reduced optical losses, thereby contributing to increase an optoelectronic device's photovoltaic conversion efficiency.